474 ELEMENTARY PHYSIOLOGY 



while, if the end connected with the cord be irritated, pain 

 immediately follows. 



When no apparent effect follows upon the irritation of 

 any nerve, it is not probable that the molecules of the 

 niirve remain unchanged. On the contrary, it would 

 appear that the same change occurs in all cases ; but a 

 motor nerve is connected with nothing that can make 

 that change apparent save a muscle, and a sensory nerve 

 with nothing that can show an effect but the central 

 nervous system. 



We have already explained (p. 468) that a fibre of the 

 anterior root of a spinal nerve is really part of a neuron 

 the cell of which is situated in the anterior horn of the 

 grey matter of the spinal cord. This being the case it is 

 not at all surprising to find that the continued life of any 

 of the efferent (motor) nerve-fibres is dependent upon the 

 continuance of their connection with the cells from which 

 they arise. That this dependence does really exist is 

 shown by the simple experiment of cutting an efferent 

 (motor) nerve, and preventing the cut ends from reuniting. 

 When this is done it is found that shortly after the 

 operation, those (peripheral) parts of the nerve beyond the 

 point of section, i.e., whose connection ivith the cells of the 

 spinal cord has been cut off, undergo what is called a 

 "degeneration." This degeneration shows itself by 

 structural changes in the nerve fibres. The medulla 

 breaks up into oily drops, the axis cylinder also breaks 

 into pieces and the neuclei of the neurilemma increase in 

 number, together with an increase in the amount of 

 granular protoplasm, which lies near them. The frag- 

 ments of the medulla are next largely absorbed and 

 disappear, and their place is taken by the protoplasm and 

 nuclei derived from the neurilemma. While these 

 structural changes are taking place, and even before 

 they become obvious, the irritability of the nerve becomes 

 gradually less, so that soon the nerve makes no response 

 to any stimulus which may be applied to it. But the 

 changes we have descx'ibed do not occur in that (central) 



